The present invention relates to an image forming apparatus of the type having a photoconductive element or similar image carrier, and an intermediate transfer belt or similar intermediate image carrier to which a toner image is transferred from the image carrier and, more particularly, to an implementation for eliminating toner filming on the intermediate image carrier and defective image transfer which degrade image quality.
It is a common practice with a copier, facsimile apparatus, printer or similar image forming apparatus, particularly a color image forming apparatus, to use an intermediate image carrier implemented as, for example, an intermediate transfer belt. In this type of apparatus, after a toner image has been transferred from the belt to a final recording medium, e.g., paper sheet, some toner is left on the belt as well as on a photoconductive element or similar image carrier. Therefore, the belt has to be cleaned to remove the remaining toner before the next image forming cycle begins. A device for cleaning the belt or the photoconductive element has customarily been implemented by a cleaning blade made of, for example, urethan rubber or a fur brush. Particularly, the device using a cleaning blade is extensively used since it is simple in mechanical arrangement and low cost.
However, the problem with the blade cleaning scheme is that as toner particles melted by heat and pressure and fine toner particles adhere to the surface of the belt, the blade cannot fully remove them alone, causing a toner filming layer to sequentially grow on the belt. The filming layer lowers the image transfer ability of the apparatus and, therefore, degrades image quality. Particularly, since image transfer is effected at least twice before the transfer of a toner image to a paper sheet or similar final recording medium, even a thin filming layer is apt to lower the transfer efficiency of the apparatus to a noticeable degree. On the other hand, when use is made of a counter blade type cleaning device, it is likely that the filming layer on the belt increases the relative coefficient of friction between the belt and the cleaning blade, thereby causing the blade to be entrained by the belt.
In light of the above, Japanese Patent Laid-Open Publication No. 2-214882 discloses a device having a cleaning roller downstream of a cleaning blade and causing it to rub the surface of an intermediate image carrier, thereby removing a filming layer. Further, Japanese Patent Laid-Open Publication No. 2-262180 proposes an improvement over the Laid-Open Publication No. 2-214882, i.e., a device having means for supporting the cleaning roller such that the roller is movable into and out of contact with the surface of the intermediate image carrier; every time image formation is repeated a predetermined number of times, the roller is brought into contact with the image carrier. Japanese Patent Laid-Open Publication N. 3-65973 teaches the use of fine particles of lubricating oil additive, although it does not pertain to the intermediate image carrier. The fine particles of lubricating oil additive are applied to the surface of a transfer drum which supports a paper sheet, thereby forming a film on the drum. This film reduces the surface energy of the transfer drum to promote the removal of a toner from the surface of the drum.
However, the device taught in the Laid-Open Publication No. 2-214882 has a drawback that the cleaning roller is apt to rub the surface of the intermediate image carrier excessively. Regarding the device of the Laid-Open Publication No. 2-262 180, although the cleaning roller is periodically brought into contact with the intermediate image carrier, the device cannot fully adapt to the environment since the growth of the filming layer depends on, among others, the conditions in which the image forming apparatus is used.
On the other hand, in a color image forming apparatus of the type having an intermediate image carrier, materials constituting an image carrier, intermediate image carrier and recording medium have to be adequately selected in respect of coefficient of friction; otherwise, defective image transfer is apt to occur. For example, so long as an image is transferred from a surface having a small coefficient of friction to a surface having a great coefficient of friction, the transfer is successful. However, when an image is transferred from the latter surface to the former surface, defective image transfer, i.e., local omission of a character image is likely to occur.